The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs, and, for these advances to be realized, similar developments in IC manufacturing are needed.
For example, behaviors of light such as diffraction, fringing, and interference become more pronounced as device size shrinks. ICs are typically assembled by layering features on a semiconductor substrate using a set of photolithographic masks. The masks have patterns formed by transmissive or reflective regions. During a photolithographic exposure, radiation such as ultraviolet light passes through or reflects off the mask before striking a photoresist coating on the substrate. The mask transfers the pattern onto the photoresist, which is then selectively removed to reveal the pattern. The substrate then undergoes processing steps that take advantage of the shape of the remaining photoresist to create circuit features on the substrate. When the processing steps are complete, photoresist is reapplied and substrate is exposed using the next mask. In this way, the features are layered to produce the final circuit.
As the minimum feature size shrinks, the nature of light causes the patterns formed on the substrate to vary from the pattern of the mask. Examples of these variances include corner rounding and edge placement errors. To correct the pattern formed on the substrate, assist features and other modifications are made to the mask in a process often referred to as optical proximity correction (OPC). Current OPC methods have delivered positive results but have not been entirely satisfactory in all regards. Thus, advances in OPC are desirable because they improve IC density, performance, and yield by allowing formation of smaller, sharper, and more uniform features.